Environmentally friendly aqueous architectural coating compositions

ABSTRACT

An environmentally friendly stable aqueous optionally structured water-resistant architectural coating composition (especially a paint) containing a film-forming binder polymer composed of a starch modified by the presence of carboxylic acid or inorganic carboxylate groups and bonded (probably grafted) to chains of copolymerized mono-ethylenically unsaturated monomers (e.g. “acrylic”, “vinyl” or “styrenic” copolymers) permitting a stable aqueous dispersion of the binder polymer to be made which retains good water-resistance in the dried paint. The modified starch allows paints to be made with less dependence on materials obtained from non-renewable resources while retaining a low water-sensitivity.

This invention relates to environmentally friendly (optionallystructured) aqueous architectural coating compositions, for examplewater-resistant paints (or varnishes or woodstains) suitable for use atambient temperatures (say up to 35° C. and usually above 0 or 5° C.) indecorating surfaces of architectural structures such as buildings orfurniture or fittings found in buildings. The invention allows starch tobe used commercially in making these aqueous compositions so reducingtheir dependency on materials obtained from non-renewable resources suchas petrochemicals. It also permits the use of smaller amounts of certainexpensive co-monomers.

All aqueous architectural paints contain film-forming binder polymer. Asa coating of liquid paint dries on a surface, this film-forming binderpolymer serves to form a film (i.e. a dried coat) of paint which bondsto the surface and also binds together all the non-volatile componentsof the paint including particularly any pigments, opacifiers andextenders present.

A wide variety of binder polymers are available, but those most commonlyused in aqueous architectural paints are of three broad types obtainedfrom mono-ethylenically unsaturated monomers and known colloquially asthe “acrylics”, the “vinyls” and the “styrenics”. The “acrylics” areusually copolymers of at least two alkyl esters of one or moremono-ethylenically unsaturated carboxylic acids (e.g. methylmethacrylate/butyl acrylate copolymer) whilst the “vinyls” usuallycomprise copolymers of a mono- vinyl ester of a saturated carboxylicacid and at least one of either an acrylic monomer or a differentmono-vinyl ester. The “styrenics” are copolymers containing styrene (ora similar mono-vinyl aromatic monomer) together with a copolymerisablemonomer which is usually an acrylic. Such binder polymers require theuse of monomers which are obtained from petrochemical feedstocks whereasnowadays it is environmentally desirable to use as much as possible ofmaterial obtainable from renewable resources. In addition, some of themonomers essential to the binder polymer contain sterically bulky groupsand are therefore relatively expensive.

Starch is one of the most abundant renewable resources because it iscommercially available from such crops as potato, wheat, maize (bothwaxy and non-waxy), rice, sago, sorghum and tapioca. Moreover, it hasbeen known since the 1960's that starch-containing film-forming polymerbinders could be made by polymerising mono-ethylenically unsaturatedmonomers at ambient pressure in the presence of aqueous dispersions ofstarch whereupon binder polymers were formed which consisted of chainsof polymerised mono-ethylenically unsaturated monomers chemicallyassociated with the starch and this reduced the amount of material inthe binder which comes from non-renewable resources. The precise natureof the association between the chains of polymerised monomers and thestarch has not yet been unequivocally determined, but it is widelybelieved that the chains graft onto the starch. Such starch-containingbinders have been used successfully in adhesives, in coating paper andin sizing textiles but instability problems led to phase separation andunpredictable increases in viscosity which meant that thestarch-containing binders were not commercially suitable for use inmaking aqueous water-resistant architectural paints.

In 1981, British Patent Specification GB 2 075 525A disclosed that longterm stability of aqueous dispersions of graft copolymers of starch and“vinyl” monomers could be improved by using a starch which had beenmodified by

a) subjecting it to acid and/or enzymatic hydrolysis either before orafter the hydrolysis and

b) reacting the starch with molecules which bring about the positiveintroduction into the starch of (preferably non-ionic or cationic)groups such as carbamylethyl, alkyl, benzyl, benzalkyl, hydroxyalkyl,cyanoalkyl, acyl or dialkylamino alkyl groups.

Very stable aqueous dispersions of starch-containing film-formingbinders were then obtained which GB 2 075 525A suggested could be usedin adhesives, paper coating, textile sizing and in the making ofwater-based (i.e. aqueous) paints, although it did not disclosewater-resistant architectural paints.

Starch-containing film-forming binders substituted with such specifiednon-ionic or cationic groups appear to have been very successful in thefields of adhesives, paper coating and textile sizing but they are notwell suited for use in making aqueous water-resistant architecturalpaints owing to a high water-sensitivity as is illustrated by the lossof abrasion resistance which occurs when dried coats of the paints aresubjected to the British Standards wet scrub resistance test describedlater. Water-sensitivity is of course a significant problem when a driedcoat of paint is to be exposed to rain or frequent water-condensation.

A secondary disadvantage is that the conventional substituted starchbinders are not very effective in creating the structure which issometimes wanted in aqueous architectural paints, especially thixotropicpaints. Structure can be created in various ways including aninteraction between a so-called “structuring agent” (which is typicallya titanium or zirconium chelate or a structuring clay) and the binderpolymer plus any cellulosic thickener which may be present in the paint.Structuring clays are clays such as laponite or bentonite clays. Thesethree types interact in some way which is not fully understood to createa gel. Gel structures reduce the tendency for a paint to drip or splashand so they make painting less messy and allow brushes and rollers to beloaded with greater volumes of paint. During application of the paint toa surface by brush or pad, the paint experiences high shear forces whichtemporarily destroy the gel structure and allow the paint to be spreadeasily over the surface. This is known as “thixotropy”.

A binder polymer for a modern aqueous architectural paint shouldpreferably be capable of manufacture in conventional plants, nearly allof which operate at ambient pressure and it should be able to formaqueous dispersions which are stable under high shear forces such as areencountered during application of a paint by brush or pad. The paintsshould be environmentally friendly and able to give dried coatings whichare sufficiently water-resistant to permit use outdoors or in humidlocations such as found in kitchens and bathrooms. Accordingly, it is anobject of this invention to provide an environmentally friendly aqueouscoating composition which contains starch which has been modified topermit the formation of stable dispersions of binder polymer but whichretains good water-resistance in the dried paint. An optional object isto provide a starch-containing binder polymer which can be used in athixotropic paint.

Accordingly, this invention provides an environmentally friendly(optionally structured and possibly thixotropic) aqueous architecturalcoating composition which includes film-forming binder polymer composedof modified starch chemically associated with chains of copolymerisedmonomers at least 93 wt % (and preferably 95 to 100 wt) of which areselected from mono-ethylenically unsaturated monomers wherein

a) the starch has been modified by the introduction of carboxylic acidgroups optionally converted to an inorganic salt,

b) up to 50 wt % of the starch-containing binder polymer is provided bythe modified starch and

c) not more than 7 mol % (preferably 0 to 3 mol %) of the copolymerisedmonomers are derived from carboxylic acid monomers.

Granular amylopectin is the predominant form of starch in most naturalsources and for this reason the starch prior to modification generallyhas a weight average molecular weight of at least 5×10⁵ and usually over1×10⁶. The preferred starches are obtained from potatoes, maize (corn)or from waxy maize. Potato and maize starch is relatively cheap andcontains over 70 wt % amylopectin whereas waxy maize starch is moreexpensive but contains even more (often over 90 wt %) of amylopectinstarch. Waxy potato starch also contains such higher levels ofamylopectin but being a genetically modified product, it is not yetauthorised for sale to the public.

The native starch is modified by subjecting it to a light oxidationsufficient to convert hydroxyl groups present in the native starch tocarboxylic acid groups. The carboxylic acid groups introduced into thestarch and also any acid groups which may be present in the copolymerchains may optionally be converted to alkali metal or ammonium salts andin practice the use of ammonium persulphate as a polymerisationinitiator will lead to the presence of ammonium salts. Alternatively theadjustment of the pH of the coating composition by means of caustic sodawill lead to sodium salts. However, ammonium salts are preferred becausethey are less harmful to water-resistance.

Light oxidation of the starch also causes a degree of chain scissionwhich likewise introduces carboxylic acid groups likewise optionallyconverted to their inorganic salts. Light oxidation is simply andcheaply performed using oxygen from for example sodium hypochlorite orhydrogen peroxide. Preferably from 1 to 10 (especially 3 to 7)carboxylic groups per 100 glucose moieties are introduced and it hasbeen found that this confers a long term stability on both the aqueousdispersions of binder and on the liquid paint which stability persistsfor at least 5 months whilst retaining a good water-resistance in thedried paint.

Carboxylic (i.e. carboxylic acid or salt) modification of the starch hasalso been found to enable the starch-containing binder polymer to beused in making structured paints. It appears that the introducedcarboxylic groups promote some sort of interaction involving chelate orclay structuring agents with the result that when the structuring agentsare introduced to the starch-containing binder, a gel structure formswhich can confer thixotropy if enough structuring agent is present.

Preferably the starch-containing binder contains less than 30 wt % (mostpreferably less than 20 wt %) of the modified starch and less than 12 wt% if very low water-sensitivity is wanted.

The starch-containing film-forming binder is preferably made at ambientpressure by dispersing the modified starch in cold water, heating thedispersion to about 70 to 95° C. and then adding conventional freeradical initiator such as ammonium persulphate to the aqueous dispersionand feeding in mono-ethylenically unsaturated monomers of the typeconventionally used to make film-forming binder polymers for aqueousarchitectural paints. If preferred, a small amount of monomer andinitiator can be added first to produce a seed and then the main chargeof monomers and initiator can be added subsequently.

As indicated earlier, examples of suitable mono-ethylenicallyunsaturated monomers include:

a) “acrylics” such as alkyl (especially methyl, ethyl, ethylhexyl andn-butyl) esters of unsaturated carboxylic acids such as acrylic ormethacrylic or fumaric acids or maleic anhydride,

b) “vinyls” such as mono-vinyl esters (especially vinyl acetate or vinyl“Versatate”¹) and

¹Vinyl “Versatate” is the vinyl ester of so-called “Versatic” acid whichis a mixture of aliphatic monocarboxylic acids each containing anaverage of 9, 10 or 11 carbon atoms and is commercially vailable fromthe Shell Chemical Company of Carrington, England.

c) “styrenics” which are usually styrene but which can be othermonovinylidene aromatics such as vinyl toluene or vinyl pyridine andwhich are usually copolymerised with comonomers such as the ethyl orethylhexyl or butyl acrylics mentioned above.

Various mono-ethylenically unsaturated acid or acid anhydride monomersmay be copolymerised with the binder monomers to increase thehydrophilic character of the binder polymer in alkaline solutions and soincrease the stability of the dispersions. However the acid comonomersmust not exceed 7 mol % of the total monomers for otherwise the binderpolymer becomes too hydrophilic to retain acceptable water-resistance.Suitable acids include unsaturated carboxylic acids and in particularacrylic or methacrylic acids and unsaturated acid anhydrides includemaleic anhydride. If copolymerisation at superatmospheric pressures iscommercially tolerable, the hydrophilic nature of the binder polymer canbe adjusted downwards by including some mono-olefin (usually someethylene) in the binder monomers.

It is desirable to choose combinations of monomers which include asterically bulky monomer so as to result in starch-containing binderpolymers in which the chain moieties have a glass transition temperatureor “Tg” as calculated using the Fox equation of below 350K andpreferably below 325K. A Tg temperature of below 270K may be necessaryif it is desired to avoid using organic coalescing solvents. Preferred“acrylic” binder copolymers include copolymers of methyl methacrylatewith butyl or 2-ethylhexyl acrylate as sterically bulky monomers andoptionally copolymerised with up to 7 mol % acrylic or methacrylic acid.Preferred “vinyl” binder copolymers include copolymers of vinyl acetatewith a bulky monomer which is usually vinyl “Versatate” or bulky acrylicmonomer as above plus the same optional acid comonomers. Preferred“styrenic” binder copolymers include copolymers of styrene with butyl or2-ethylhexyl acrylate serving as the bulky monomers with optionally upto 7 mol % acid comonomers as above. Tg will be increased unacceptablyby excessive crosslinking and so it is essential not to incorporate morethan 7 wt % of a conjugated diene into the binder polymer and it is verymuch preferred to avoid any such diene. The presence of the free radicalinitiator at 70 to 90° C. causes polymerisation of the unsaturatedmonomers to produce chains which are in some way chemically associatedwith the modified starch. Such association has been found to permit theuse of a lower proportion of the sterically bulky monomers (which arerelatively expensive) without loss of the ability to apply the paint atambient temperatures.

Surprisingly, it has been found that the water-resistance of the driedcoat of paint can be further increased by adding anionic surfactant tothe aqueous dispersion of the modified starch even though surfactantsusually cause water-sensitivity.

An aqueous dispersion of the starch-containing film-forming binder iseasily converted into a coating composition by stirring it together withall the other components of the composition except for any chelatestructuring agent. If a structured composition structured by means of achelate is wanted, the chelate should be stirred in just before thecomposition is filled into cans so that the gel structure develops inthe can. The most significant of the other components are pigments andopacifiers (usually rutile titanium dioxide or voided organic polymerparticles) and also extenders which are solid particles which serve tospace apart the pigments and opacifiers. Typical extenders are chalk,limestone, kaolin and talc. Silica may also be present as a mattingagent. Whilst this invention is of most importance in making paints, itis also possible to omit the opacifier to produce a varnish orwoodstain. The coating compositions preferably have a “Rotothinner” (lowshear) viscosity of from 0.15 to 2.0 pascal.sec and a cone and plate(high shear) viscosity of from 0.03 to 0.5 pascal.sec all measured at25° C. using techniques specified later in this specification. Athixotropic structured composition preferably has a gel strength at 1week of at least 50 g.cm.

This invention also provides a process for making an environmentallyfriendly (optionally structured and possibly thixotropic) aqueous paintas described above which includes film-forming binder polymer composedof modified starch chemically associated with chains of copolymerisedmonomers at least 93 wt % (and preferably 95 to 100 wt %) of which areselected from mono-ethylenically unsaturated monomers wherein theprocess includes the steps of

a) modifying a starch by lightly oxidising it to introduce carboxylicacid groups, optionally converted to an inorganic salt,

b) adding free radical initiator to an aqueous dispersion of themodified starch and feeding the unsaturated monomers into the dispersionpreferably in the presence of anionic surfactant,

c) subjecting the dispersion to a temperature which causescopolymerisation of the monomers to produce chains of copolymerisedmonomers chemically associated with the modified starch so in turncreating the starch-containing film-forming binder,

d) mixing the binder polymer so produced with other components of thecomposition,

e) choosing the ratio of modified starch to unsaturated monomers so asto ensure that the weight of modified starch in the starch-containingbinder does not exceed 50 wt % of the weight of the starch-containingbinder and

f) choosing a ratio of monomers such that not more than 7 mol %(preferably 0 to 3 mol %) of the copolymerised monomers are derived fromcarboxylic acid monomers.

This process can be successfully performed at ambient pressure providedthat the co-monomers do not include monomers such as ethylene or 1,3-butadiene which are gaseous under ambient pressures at theconventional copolymerisation temperatures.

The invention is further illustrated by the following Examples of whichExamples A to C are comparative. In the examples:

Water-resistance was assessed in terms of loss of wet scrub resistancemeasured as the loss of paint in grams as determined according toBritish Standard BS 7719 1994 but when performed on dried coats of paintwhich are only 1 week old.

“Rotothinner” (low shear) viscosity is measured using the Sheen“Rotothinner” which is described in the Sheen Data Sheet on“Rotothinners” available from the address given below. Againmeasurements are performed at 25° C. “Rotothinner” viscosity correspondsto the viscosity of the paint when being poured from a can or loadedonto a brush.

Cone and Plate (high shear) viscosity is measured according to ASTM TestD 4287-88 at 25° C. Cone and Plate high shear viscosity corresponds tothe viscosity of the paint when being applied by brush.

Gel Strength is measured using Sheen Gel Strength Tester supplied bySheen Instruments Limited of Kingston Surrey England and described intheir Data sheet Ref. 414. The measurement is performed at 25° C. usinga 4×2 cm paddle.

EXAMPLE 1 Preparation of a Binder and a Paint when the Starch is aModified Potato Starch

a) Preparation of the Binder

64 g of a commercial modified starch which was a proprietary lightlyoxidised granular potato starch believed to contain from 1 to 10carboxylic acid groups per 100 glucose moieties was added via a funnelinto 440 g cold (20° C.) water in a reflux flask together with 1.2 g ofsodium acetate buffer. The funnel was washed with a further 44 g waterwhich was added to the flask. The flask was heated with stirring to 85°C. for 30 minutes to solubilise the starch.

The contents of the flask were allowed to cool to 67° C. and then 37 gof vinyl acetate monomer mixed with 9 g vinyl “Versatate” monomer wereadded as binder monomers and the contents were stirred for 10 minutes.Next, 1.2 g of ammonium persulphate initiator dissolved in water wereadded to the contents which were then heated to 80° C. with stirringover a period of 35 minutes to produce a seed polymer. Then thetemperature was raised to 85° C. and 334 g of vinyl acetate, 83 g vinyl“Versatate” and 1.3 g ammonium persulphate in water were added graduallywith stirring to the flask over a period of 3 hours. During this time,the major proportion of the vinyl acetate/vinyl “Versatate” film-formingbinder copolymer formed and grafted onto the modified starch. Thecarboxylic acid groups on the starch had been converted to the ammoniumsalt owing to the presence of ammonium persulphate.

Finally a further 0.54 g of ammonium persulphate in water were added tothe flask over a period of 10 minutes with continuing stirring at 85°C., in order to complete the copolymerisation and grafting process. Thecontents of the flask were then allowed to cool to 70° C. and 1 gt-butyl hydroperoxide in water was added. Further cooling to 60° C. wasallowed and 1 g sodium metabisulphite was added followed by cooling toambient (18° C.) temperature. It was found that a stable aqueousdispersion of oxidised potato starch-containing vinyl acetate/vinyl“Versatate” film-forming binder copolymer containing 10 wt % of theoxidised starch had been produced. The “vinyl” component of thestarch-containing binder polymer had a theoretical Glass TransitionTemperature (Tg) as determined by the Fox equation of 303° K. and thewhole process was performed at ambient pressure.

b) Preparation of a Structured Paint

The binder was converted into a structured architectural paint asfollows

A millbase was made up by stirring 69.7 g particulate rutile titaniumdioxide, 1.6 g bentonite structuring clay and 15.3 g particulate calciumcarbonate extender into 88 g of water containing 1.1 g ofpolycarboxylate dispersant and 5.3 g benzyl alcohol. Stirring wasperformed using a high speed stirrer operating at 3 000 rpm for 20minutes. Stirring was then slowed to 300 rpm. 52 g water and 3.2 gcellulosic thickener were added gradually during a period of fiveminutes followed by stirring at 3000 rpm for 10 minutes.

Ammonia was added to 230 g of an aqueous dispersion of the binder so asto produce a pH of 8. The millbase was then added gradually to thedispersion of binder copolymer over a period of 30 minutes with stirringat 300 rpm. After addition of the millbase, 30 g “Ropaque” white voidedorganic pigment were stirred into the mixture followed by 2.5 g ofzirconium chelate structuring agent. The mixture was allowed to standfor 15 minutes and then was poured into cans where it developed a gelstructure and became an aqueous structured architectural paint. The gelstrength of the paint after 1 week and its viscosities were measured andthe results are shown in Table 1 together with an assessment of itswater sensitivity measured using the scrubbing technique of BritishStandard 7719:1994.

EXAMPLE 2 And Comparative Examples A to C

The Use of other Starch Types

The procedure of Example 1 was repeated except the oxidised potatostarch was replaced in turn by the following alternative starch types:

Example 2 Oxidised waxy maize starch

Comparative Example A Enzyme degraded native potato starch.

Comparative Example B Starch which had been subjected to substitution byhydroxypropyl groups as suggested by GB 2 075 525A.

TABLE 1 PROPERTIES OF PAINTS MADE USING DIFFERENT TYPES OF MODIFIEDSTARCHES Rotothinner BS Water Viscosity Cone & Plate Scrub Stability GelStrength (Low Shear) (High Shear) Resistance after 5 Example Starch TypePa · sec Pa · sec Viscosity Pa · sec mg/cm² Months 1 Oxidised Potato 900.6 0.1 3.3 Stable 2 Oxidised Waxy Maize 70 0.5 0.1 2.8 Stable 3Oxidised Potato plus 130 0.5 0.1 1.6 Stable anionic surfactant A Enzymedegraded 75 0.5 0.1 3.0 Flocculated potato B Hydroxy propylated 30 0.5Unstable 7.5 Unstable Potato under high shear C *Conventional 150 0.60.1 1.9 Stable Gel Strengths and Viscosities measured after 1 week *Theconventional binder was a Vinyl Acetate/20 wt % Vinyl “Versatate”copolymer

From Table 1, it can be seen that the hydroxypropylatedstarch-containing binder polymer could produce a gel strength of only 30g.cm, showed a high water sensitivity and became unstable under highshear such as is encountered when a paint is brushed onto a surface. Thedispersions and paints containing enzyme degraded potato starch wereunstable and flocculated after 5 months.

Comparative Example C

An aqueous thixotropic paint was made up according to Part b) of Example1 except that the starch-containing binder polymer was replaced by aconventional vinyl acetate/vinyl “Versatate” copolymer containing 20 wt% of vinyl “Versatate”.

EXAMPLE 3

The Use of Anionic Surfactant

Preparation of Binder Polymer

55 g of the modified potato starch of Example 1 was added via a funnelto 398 g cold (20° C.) water in a reflux flask together with 1.2 g ofsodium acetate and 4.9 g of a proprietory anionic surfactant. Thecontents of the flask were heated to 85° C. with stirring and 3.1 g ofammonium persulphate in water were added. The temperature was maintainedwith stirring at 85° C. for 30 minutes during which time milddegradation occurred. Then the temperature was allowed to fall to 65° C.

46 g of the vinyl acetate/vinyl “Versatate” mixture of Example 1 werefed to the contents which were held at 65° C. for 10 minutes. 0.8 g ofammonium persulphate in water were added and the temperature raised to80° C. with stirring over 35 minutes to produce a seed copolymer similarto that of Example 1 and again the carboxylic acid groups on the starchwere converted to the ammonium salt because of the use of ammoniumpersulphate.

Having produced the seed copolymer, the procedure of Example 1 was thenresumed except that 328 g of vinyl acetate, 82 g of vinyl “Versatate”and 4.9 g of the anionic surfactant were added to produce a main bulk ofcopolymer which was slightly (but not materially) different to that ofExample 1. The main bulk of the copolymer comprised 80 wt % vinylacetate and 20 wt % vinyl “Versatate”. It has been found subsequentlythat the proportion of vinyl “Versatate” can be reduced to 15 wt % witha corresponding increase to 85 wt % vinyl acetate without sufficientlyaffecting the Tg of the binder polymer to hinder application at ambienttemperatures.

The starch-containing binder polymer obtained was then used to make astructured paint as in Example 1.

A dried coat of the paint produced was found to be more opaque than thatof Example 1 and its water-sensitivity was found to have fallensubstantially to give a British Scrub reading of only 1.6 mg/cm² after 1week.

What is claimed is:
 1. An environmentally friendly aqueous architectural coating composition containing a film-forming binder polymer, comprising: a modified starch chemically associated with chains of copolymerised ethylenically monomers, where at least 93% by weight of the ethylenically unsaturated monomers are mono-ethylenically unsaturated monomers, and up to 7% by weight of the ethylenically unsaturated monomers are carboxylic acid monomers; the modified starch being unmodified starch with hydroxyl groups, where the hydroxyl groups have been modified by oxidation to form carboxyl acid groups in the modified starch; and where the modified starch and the copolymerised ethylenically unsaturated monomers are chemically associated by forming an aqueous dispersion of the modified starch and the ethylenically unsaturated monomers, and copolymerising the ethylenically unsaturated monomers to form the film-forming binder polymer of associated copolymerised monomers and modified starch, where the film-forming binder comprises up to 50% by weight of said modified starch.
 2. A coating composition according to claim 1 wherein the modified starch contains 1 to 10 carboxylic acid or salt groups per 100 glucose moieties.
 3. A coating composition according to claim 1 wherein the modified starch is an oxidised potato or an oxidised maize starch.
 4. A coating composition according to claim 1 wherein the composition is structured.
 5. A coating composition according to claim 4 wherein the composition is structured by means of a titanium or zirconium chelate or a structuring clay.
 6. The coating composition of claim 1 wherein the ethylenically unsaturated monomers comprise 95% to 100% by weight of the mono-ethylenically unsaturated monomers.
 7. The coating composition of claim 1 wherein 0 to 3% by weight of the ethylenically unsaturated monomers comprise carboxylic monomers.
 8. The coating composition of claim 1 where the carboxyl groups formed on the modified starch are converted to alkali metal salts.
 9. The coating composition of claim 1 where the carboxyl groups formed on the modified starch are converted to ammonium salts.
 10. The coating composition of claim 1 where the carboxyl groups formed on the modified starch are converted to sodium salts.
 11. The coating composition of claim 1 where the film-forming binder polymer comprises by weight less than 30% of the modified starch.
 12. The coating composition of claim 1 where the film-forming binder polymer comprises by weight less than 20% of the modified starch. 